This invention relates generally to the repair of fluid carrying tubes such as those carrying natural gas, water or steam. More particularly, this invention has application to the repair of a steam generator.
Steam generators are an integral part of a power generating station. Over a period of time, the steam generator, or any fluid carrying tube for that matter, may develop leaks due to the combined corrosive effects of the fluid medium and the environment in which the steam generator or fluid carrying tube is situated. Whenever a leak is found, it must be stopped in some manner. One method is to plug the damaged tube. This method, of course, leads to reduced operating capacity. When enough damaged tubes become plugged, the power station must be shut down for retubing of the steam generator. Due to the time and expense involved, it would be desirable to avoid shutdown for as long as possible.
An alternative method is the repair in situ of the damaged tube. If the tube is repaired rather than plugged, operating capacity is not substantially reduced. A common embodiment of this alternative method is the insertion of a repair sleeve adjacent to the leak in the damaged pipe. When the repair sleeve is placed in position, it is desirable that it be firmly retained there. This is desirable for two reasons. The first is that any dislodgement of the repair sleeve would be totally unacceptable from the standpoint of the operation of the steam generator. The second is that firm retention promotes a leak-tight fit.
It is also desirable that the repair sleeve apparatus be capable of being deployed quickly and efficiently so as to avoid expensive labor costs. It is most desirable that the repair sleeve operation be subject to automation as by a remote manipulator.
Solutions as heretofore proposed in the prior art have failed to achieve these aims.
In U.S. Pat. No. 1,471,184 to Miles, one method of inserting and retaining a repair sleeve is illustrated. Miles shows expanders in either end of the sleeve which are simultaneously drawn together by a mechanical apparatus including a draw member. Packers are compressed between the sleeve and the expanders to provide sealing. The draw member is operated by turning a wing nut which can be tedious and time consuming. Removal of the draw member can also be burdensome.
In the U.S. Pat. No. 4,069,573 to Rogers, Jr. et al, the repair sleeve and damaged tube are plastically deformed by an hydraulic apparatus. According to the disclosure, an interference fit results between the repair sleeve and joint but due to their essentially equivalent elastic constants, a slight mechanical clearance, i.e., leakage, may exist. This is an unsatisfactory situation for two reasons. The first is that this apparatus is not suitable for situations where a leak proof fit is necessary. The second is that mechanical deformation of the damaged tube is undesirable and is even more undesirable when the damaged tube is constrained within a tube sheet.
In U.S. Pat. No. 2,756,779 to Tratzik et al. an expanding mandrel is utilized to expand deformable ferrules on either end of the repair sleeve. This apparatus is unsatisfactory because of the time and manpower necessary to secure the repair sleeve. The distal end of the repair sleeve is expanded first and then the whole apparatus must be reset before expanding the near end of the repair sleeve. Such a procedure is expensive in operation and is impractical when repairing a large number of tubes. It is also possible that due to the elastic component of the ferrule springing back, a leak tight seal may not be achieved. Other similar unwieldy mechanical arrangements are described in U.S. Pat. Nos. 2,731,041, 2,784,627, and 2,829,675, all to Mueller et al.
Mechanical arrangements where two expanding mandrels are drawn toward one another are shown in U.S. Pat. No. 2,517,626 to Berg and U.S. Pat. No. 4,114,654 to Richardson. Both of these arrangements would be unsatisfactory for use in securing repair sleeves since all of the individual components remain in the damaged tube, thereby severely restricting the fluid flow to an untolerable level.
U.S. Pat. No. 4,114,654 discloses a plug for a tube which comprises a deformable tubular sleeve and two tapered members positioned partially within the sleeve. The members can be drawn together by a screw thread arrangement so as to deform the sleeve outwardly into engagement with the tube to be plugged.
In conjunction with the aim to automate the sleeve repair operation, it would be desirable to deploy the sleeve repair apparatus with as little manipulation as possible.
Materials capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of heat alone, it can be caused to revert, or to attempt to revert, from its heat-unstable configuration to its original, heat-stable configuration, i.e. it "remembers" its original shape.
Among metallic alloys, the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s and the temperature at which it finishes M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete) the deformed object will begin to return to its original configuration.
Thus, in a sense, shape memory alloys can be self-actuating when exposed to a change in temperature. When these shape memory alloys promote the movement of a cooperating part, they become self-actuating drivers.
The use of shape memory alloys as self-actuating drivers has heretofore been limited to devices such as a coupling, (U.S. Pat. No. 4,149,911 to Clabburn), a steam generator tube plug (U.S. Pat. No. 3,900,939 to Greacan) and a prestressed structural joint member (U.S. Pat. No. 4,294,559 to Schutzler). As far as applicant is aware, no article has been disclosed in which the shape memory effect is utilized to axially contract a longitudinally dimensioned draw means.